In one of the earliest classifications of diatoms, the individual cell received less consideration than the nature of the filament or thallus in which many species occur in the first stages of their growth. Those, however, which exist in colonies at first are, sooner or later, broken up into separate frustules, either before or at the time of their maturity or previous to conjugation, while very many species are never seen except in a free state. The union of frustules, therefore, is of secondary importance and the group must be considered as filamentous or unicellular algæ. Their relation to other algæ is not well determined. Among the _Desmidiaceæ_, a family of the order _Conjugales_, of the class _Chlorophyceæ_, the cells are in many forms divided by a constriction into symmetrical halves. The Conjugales are starch forming, with walls of cellulose. In the Diatomaceæ the starch is replaced by oil globules, while the walls of cellulose are more or less filled with a deposit of silica. The Conjugales, however, reproduce by zygospores and usually contain pyrenoids, as may be seen in the parietal chromatophores of _Spirogyra_. In the class _Heterokontæ_ we have the reserve material in the form of oil, instead of starch, but there are no pyrenoids. To this class belongs the order _Confervaceæ_, in which the cells are unicellular or filamentous, and to which all of the Diatomaceæ were referred. While, therefore, Diatomaceæ have a close affinity to the Desmidiaceæ and to the Confervaceæ, the determination of their origin, one from another, or from a common ancestral type, appears to be a matter of conjecture. {9}MORPHOLOGY AND DEVELOPMENT THE CELL The cell membrane is composed of two usually equal parts, each of which consists of a valve and a girdle or zone formed of cellulose modified by silica deposited in an insoluble state from a very dilute aqueous solution. The valves are more siliceous and robust than the girdle. Both are in most species easily separable, or at least the bands of the girdle which may be more or less closely fastened to the valves have a motion over each other permitting the cell to enlarge at pleasure. The longitudinal diameter of the cell, or the distance between the centres of the two valves, will vary according to the convexity of the valve and the age of the frustule which may be often determined by the width or number of the girdle bands. These, owing to their diversity of form and arrangement, will be further described under the generic diagnoses. The siliceous cell-wall is covered on the outside by a layer of protoplasm called the coleoderm. This layer may be quite thin and evident only when treated with fuchsin or Bismarck brown, or it may be of considerable thickness. The cell contains the cytoplasma, protoplasm, cell-sap, endochrome, pyrenoids, oil globules and nucleus, together with certain other less understood bodies.